We report on the filaments that develop self-consistently in a new numerical simulation of cloud formation by colliding flows. As in previous studies, the forming cloud begins to undergo gravitational collapse because it rapidly acquires a mass much
larger than the average Jeans mass. Thus, the collapse soon becomes nearly pressureless, proceeding along its shortest dimension first. This naturally produces filaments in the cloud, and clumps within the filaments. The filaments are not in equilibrium at any time, but instead are long-lived flow features, through which the gas flows from the cloud to the clumps. The filaments are long-lived because they accrete from their environment while simultaneously accreting onto the clumps within them; they are essentially the locus where the flow changes from accreting in two dimensions to accreting in one dimension. Moreover, the clumps also exhibit a hierarchical nature: the gas in a filament flows onto a main, central clump, but other, smaller-scale clumps form along the infalling gas. Correspondingly, the velocity along the filament exhibits a hierarchy of jumps at the locations of the clumps. Two prominent filaments in the simulation have lengths ~15 pc, and masses ~600 Msun above density n ~ 10^3 cm-3 (~2x10^3 Msun at n > 50 cm-3). The density profile exhibits a central flattened core of size ~0.3 pc and an envelope that decays as r^-2.5, in reasonable agreement with observations. Accretion onto the filament reaches a maximum linear density rate of ~30 Msun Myr^-1 pc^-1.
We present a numerical study of the evolution of molecular clouds, from their formation by converging flows in the warm ISM, to their destruction by the ionizing feedback of the massive stars they form. We improve with respect to our previous simulat
ions by including a different stellar-particle formation algorithm, which allows them to have masses corresponding to single stars rather than to small clusters, and with a mass distribution following a near-Salpeter stellar IMF. We also employ a simplified radiative-transfer algorithm that allows the stellar particles to feed back on the medium at a rate that depends on their mass and the local density. Our results are as follows: a) Contrary to the results from our previous study, where all stellar particles injected energy at a rate corresponding to a star of ~ 10 Msun, the dense gas is now completely evacuated from 10-pc regions around the stars within 10-20 Myr, suggesting that this feat is accomplished essentially by the most massive stars. b) At the scale of the whole numerical simulations, the dense gas mass is reduced by up to an order of magnitude, although star formation (SF) never shuts off completely, indicating that the feedback terminates SF locally, but new SF events continue to occur elesewhere in the clouds. c) The SF efficiency (SFE) is maintained globally at the ~ 10% level, although locally, the cloud with largest degree of focusing of its accretion flow reaches SFE ~ 30%. d) The virial parameter of the clouds approaches unity before the stellar feedback begins to dominate the dynamics, becoming much larger once feedback dominates, suggesting that clouds become unbound as a consequence of the stellar feedback. e) The erosion of the filaments that feed the star-forming clumps produces chains of isolated dense blobs reminiscent of those observed in the vicinity of the dark globule B68.
We investigate the properties of star forming regions in a previously published numerical simulation of molecular cloud formation out of compressive motions in the warm neutral atomic interstellar medium, neglecting magnetic fields and stellar feedba
ck. In this simulation, the velocity dispersions at all scales are caused primarily by infall motions rather than by random turbulence. We study the properties (density, total gas+stars mass, stellar mass, velocity dispersion, and star formation rate) of the cloud hosting the first local, isolated star formation event in the simulation and compare them with those of the cloud formed by a later central, global collapse event. We suggest that the small-scale, isolated collapse may be representative of low- to intermediate-mass star-forming regions, while the large-scale, massive one may be representative of massive star forming regions. We also find that the statistical distributions of physical properties of the dense cores in the region of massive collapse compare very well with those from a recent survey of the massive star forming region in the Cygnus X molecular cloud. The star formation efficiency per free-fall time (SFE_ff) of the high-mass SF clump is low, ~0.04. This occurs because the clump is accreting mass at a high rate, not because its specific SFR (SSFR) is low. This implies that a low value of the SFE_ff does not necessarily imply a low SSFR, but may rather indicate a large gas accretion rate. We suggest that a globally low SSFR at the GMC level can be attained even if local star forming sites have much larger values of the SSFR if star formation is a spatially intermittent process, so that most of the mass in a GMC is not participating of the SF process at any given time.
